Here's my own list of why marriage is on the decline, and it has to do with children:
Anybody who is honest with themselves will know that, biologically, men are simply not all that excited about kids. Not the diaper changing, the crying, the babying, or the daily grind of taking care of them. That's just a fact.
Why have men hung around at all over the eons? It was the culmination of factors that no longer exist.
1. The babying was done by the mother. Today, you are expected to do the babying--and if you don't like it--you are deemed a horrible, uninvolved father.